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           #include <sys/types.h>
           #include <sys/wait.h>
           pid_t wait(int *status);
           pid_t waitpid(pid_t pid, int *status, int options);
           int waitid(idtype_t idtype, id_t id, siginfo_t *infop, int options);
                           /* This is the glibc and POSIX interface; see
                              NOTES for information on the raw system call. */
       Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
               _SVID_SOURCE || _XOPEN_SOURCE >= 500 ||
               || /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L


           All of these system calls are used to wait for state changes in a child
           of  the  calling  process, and obtain information about the child whose
           state has changed.  A state change is considered to be: the child  ter-
           minated; the child was stopped by a signal; or the child was resumed by
           a signal.  In the case of a terminated child, performing a wait  allows
           the  system  to  release  the resources associated with the child; if a
           wait is not performed, then the terminated child remains in a  "zombie"
           state (see NOTES below).
           If  a  child has already changed state, then these calls return immedi-
           ately.  Otherwise they block until either a child changes  state  or  a
           signal  handler interrupts the call (assuming that system calls are not
           automatically restarted using the SA_RESTART flag of sigaction(2)).  In
           the  remainder  of this page, a child whose state has changed and which
           has not yet been waited upon by one of these  system  calls  is  termed
       wait() and waitpid()
           The  wait() system call suspends execution of the calling process until
           one of its children terminates.  The call wait(&status)  is  equivalent
               waitpid(-1, &status, 0);
           The  waitpid()  system  call  suspends execution of the calling process
           until a child specified by pid argument has changed state.  By default,
           waitpid() waits only for terminated children, but this behavior is mod-
           ifiable via the options argument, as described below.
           The value of pid can be:
           < -1   meaning wait for any child process whose  process  group  ID  is
           WUNTRACED   also return if a child has  stopped  (but  not  traced  via
                       ptrace(2)).   Status for traced children which have stopped
                       is provided even if this option is not specified.
           WCONTINUED (since Linux 2.6.10)
                       also return if a stopped child has been resumed by delivery
                       of SIGCONT.
           (For Linux-only options, see below.)
           If status is not NULL, wait() and waitpid() store status information in
           the int to which it points.  This integer can  be  inspected  with  the
           following  macros  (which take the integer itself as an argument, not a
           pointer to it, as is done in wait() and waitpid()!):
                  returns true if the child terminated normally, that is, by call-
                  ing exit(3) or _exit(2), or by returning from main().
                  returns  the  exit  status  of  the child.  This consists of the
                  least significant 8 bits of the status argument that  the  child
                  specified  in  a  call to exit(3) or _exit(2) or as the argument
                  for a return statement in main().  This macro should be employed
                  only if WIFEXITED returned true.
                  returns true if the child process was terminated by a signal.
                  returns  the  number of the signal that caused the child process
                  to terminate.  This macro should be employed only if WIFSIGNALED
                  returned true.
                  returns  true  if  the  child  produced a core dump.  This macro
                  should be employed only  if  WIFSIGNALED  returned  true.   This
                  macro  is  not specified in POSIX.1-2001 and is not available on
                  some UNIX implementations (e.g., AIX,  SunOS).   Only  use  this
                  enclosed in #ifdef WCOREDUMP ... #endif.
                  returns  true  if the child process was stopped by delivery of a
                  signal; this is possible only if the call was  done  using  WUN-
                  TRACED or when the child is being traced (see ptrace(2)).
                  returns the number of the signal which caused the child to stop.
                  This macro should be employed only if WIFSTOPPED returned  true.
                  Wait for any child whose process group ID matches id.
           idtype == P_ALL
                  Wait for any child; id is ignored.
           The child state changes to wait for are specified by ORing one or  more
           of the following flags in options:
           WEXITED     Wait for children that have terminated.
           WSTOPPED    Wait  for  children that have been stopped by delivery of a
           WCONTINUED  Wait for  (previously  stopped)  children  that  have  been
                       resumed by delivery of SIGCONT.
           The following flags may additionally be ORed in options:
           WNOHANG     As for waitpid().
           WNOWAIT     Leave  the child in a waitable state; a later wait call can
                       be used to again retrieve the child status information.
           Upon successful return, waitid() fills in the following fields  of  the
           siginfo_t structure pointed to by infop:
           si_pid      The process ID of the child.
           si_uid      The  real  user ID of the child.  (This field is not set on
                       most other implementations.)
           si_signo    Always set to SIGCHLD.
           si_status   Either the exit status of the child, as given  to  _exit(2)
                       (or exit(3)), or the signal that caused the child to termi-
                       nate, stop, or continue.  The si_code field can be used  to
                       determine how to interpret this field.
           si_code     Set   to   one  of:  CLD_EXITED  (child  called  _exit(2));
                       CLD_KILLED (child  killed  by  signal);  CLD_DUMPED  (child
                       killed  by  signal,  and  dumped  core); CLD_STOPPED (child
                       stopped by signal); CLD_TRAPPED (traced child has trapped);
                       or CLD_CONTINUED (child continued by SIGCONT).
           If  WNOHANG  was  specified  in options and there were no children in a
           waitable state, then waitid() returns 0 immediately and  the  state  of
           the siginfo_t structure pointed to by infop is unspecified.  To distin-
           guish this case from that where a child was in a waitable  state,  zero
           out  the  si_pid field before the call and check for a nonzero value in
           this field after the call returns.


           ECHILD (for  wait()) The calling process does not have any unwaited-for
           ECHILD (for waitpid() or waitid()) The process specified by pid  (wait-
                  pid())  or  idtype  and id (waitid()) does not exist or is not a
                  child of the calling process.  (This can happen  for  one's  own
                  child if the action for SIGCHLD is set to SIG_IGN.  See also the
                  Linux Notes section about threads.)
           EINTR  WNOHANG was not set and an unblocked signal  or  a  SIGCHLD  was
                  caught; see signal(7).
           EINVAL The options argument was invalid.


           SVr4, 4.3BSD, POSIX.1-2001.


           A  child  that  terminates, but has not been waited for becomes a "zom-
           bie".  The kernel maintains a minimal set of information about the zom-
           bie  process  (PID,  termination status, resource usage information) in
           order to allow the parent to later perform a wait to obtain information
           about  the  child.   As long as a zombie is not removed from the system
           via a wait, it will consume a slot in the kernel process table, and  if
           this  table fills, it will not be possible to create further processes.
           If a parent process terminates, then its "zombie" children (if any) are
           adopted  by  init(8), which automatically performs a wait to remove the
           POSIX.1-2001 specifies that if the disposition of  SIGCHLD  is  set  to
           SIG_IGN or the SA_NOCLDWAIT flag is set for SIGCHLD (see sigaction(2)),
           then children that terminate do not become zombies and a call to wait()
           or  waitpid()  will  block until all children have terminated, and then
           fail with errno set to ECHILD.  (The original POSIX standard  left  the
           behavior  of  setting  SIGCHLD  to SIG_IGN unspecified.  Note that even
           though the default disposition of SIGCHLD is "ignore", explicitly  set-
           ting  the disposition to SIG_IGN results in different treatment of zom-
           bie process children.)  Linux 2.6 conforms to this specification.  How-
           ever,  Linux  2.4 (and earlier) does not: if a wait() or waitpid() call
           is made while SIGCHLD is being ignored, the call behaves just as though
           SIGCHLD were not being ignored, that is, the call blocks until the next
           child terminates and then returns the process ID  and  status  of  that
       Linux notes
           In  the  Linux kernel, a kernel-scheduled thread is not a distinct con-
           struct from a process.  Instead, a thread is simply a process  that  is
           created  using  the  Linux-unique  clone(2) system call; other routines
           such as the  portable  pthread_create(3)  call  are  implemented  using
           clone(2).  Before Linux 2.4, a thread was just a special case of a pro-
           cess, and as a consequence one thread could not wait on the children of
           another  thread, even when the latter belongs to the same thread group.
                  Wait for all children, regardless  of  type  ("clone"  or  "non-
           __WNOTHREAD (since Linux 2.4)
                  Do  not  wait  for  children of other threads in the same thread
                  group.  This was the default before Linux 2.4.
           The raw waitid() system call takes a  fith  argument,  of  type  struct
           rusage *.   If  this  argument  is  non-NULL, then it is used to return
           resource usage information about the  child,  in  the  same  manner  as
           wait4(2).  See getrusage(2) for details.


           According  to POSIX.1-2008, an application calling waitid() must ensure
           that infop points to a siginfo_t structure (i.e., that it is a non-null
           pointer).   On  Linux, if infop is NULL, waitid() succeeds, and returns
           the process ID of the  waited-for  child.   Applications  should  avoid
           relying on this inconsistent, nonstandard, and unnecessary feature.


           The  following  program  demonstrates the use of fork(2) and waitpid().
           The program creates a child process.  If no  command-line  argument  is
           supplied  to  the  program, then the child suspends its execution using
           pause(2), to allow the user to send signals to the  child.   Otherwise,
           if  a  command-line  argument is supplied, then the child exits immedi-
           ately, using the integer supplied on the command line as the exit  sta-
           tus.   The parent process executes a loop that monitors the child using
           waitpid(), and uses the W*() macros described above to analyze the wait
           status value.
           The following shell session demonstrates the use of the program:
               $ ./a.out &
               Child PID is 32360
               [1] 32359
               $ kill -STOP 32360
               stopped by signal 19
               $ kill -CONT 32360
               $ kill -TERM 32360
               killed by signal 15
               [1]+  Done                    ./a.out
       Program source
           #include <sys/wait.h>
           #include <stdlib.h>
           #include <unistd.h>
           #include <stdio.h>
                       pause();                    /* Wait for signals */
               } else {                    /* Code executed by parent */
                   do {
                       w = waitpid(cpid, &status, WUNTRACED | WCONTINUED);
                       if (w == -1) {
                       if (WIFEXITED(status)) {
                           printf("exited, status=%d\n", WEXITSTATUS(status));
                       } else if (WIFSIGNALED(status)) {
                           printf("killed by signal %d\n", WTERMSIG(status));
                       } else if (WIFSTOPPED(status)) {
                           printf("stopped by signal %d\n", WSTOPSIG(status));
                       } else if (WIFCONTINUED(status)) {
                   } while (!WIFEXITED(status) && !WIFSIGNALED(status));


           _exit(2),  clone(2),  fork(2),  kill(2),  ptrace(2), sigaction(2), sig-
           nal(2), wait4(2), pthread_create(3), credentials(7), signal(7)

    Linux 2013-09-04 WAIT(2)


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